Usually, the lag of the website is proportional with the number of features
If a website has too many features, then it will lag more
This is because it has to process more data. For example, if you go to a website designed 10 years ago you’ll see it loads at the moment. This is because the UI and the features inside it is very low
But during the past of the years, lots of new things were added, and with the progress of technology machines are more powerful to process those changes
If you access with a computer of 10 years ago to the latest version of Google Cloud, it will take too much longer to load than if you do with a computer of 1 year ago
I don’t know if I’ve explained myself, but what I want to say is that with the past of the time websites required and will require more and more resources in order to show the latest features. Material Design has lots of animations which browsers need to process
AppInventor runs much more smootly than Makeroid because their UI hasn’t changed since years ago, and their animations are very simple, plus they don’t have Material Design. But compare AppInventor design with Makeroid one: IMHO I think that Makeroid feels much more modern, am I right?